1. Field of the Invention
The present invention relates to an AC-DC converter that receives an input of an alternating-current power supply voltage and that outputs a direct-current voltage, and in particular relates to a PFC (power factor correction) converter that improves the power factor.
2. Description of the Related Art
General switching power supply devices, which receive a commercial alternating-current power supply as an input power supply, rectify and smooth the commercial alternating-current power supply and thereby convert the commercial alternating-current power supply into a direct-current voltage, and since the direct-current voltage is then switched using a DC-DC converter, the input current becomes discontinuous and significantly distorted from a sine wave. This causes generation of harmonic currents.
In Japan, Europe, and elsewhere, there are regulations concerning harmonic currents, classified in accordance with usage, input power and the like. Accordingly, power factor improvement circuit (PFC) converters, such as the ones described in Japanese Unexamined Patent Application Publication No. 2006-513682 and Japanese Unexamined Patent Application Publication No. 2006-187140, are provided in the power supplies of domestic electrical appliances that are to comply with such regulations.
In general, a PFC converter is formed by a chopper circuit and operates such that the waveform of the input current comes to have a similar shape to the waveform of the input voltage, in other words, such that the waveform of the input current comes to have the shape of a sine wave of the same phase. Accordingly, harmonic currents are suppressed to a certain level or less.
Here, an example configuration of the PFC converter described in Japanese Unexamined Patent Application Publication No. 2006-513682 will be described on the basis of FIG. 1. In the PFC converter of FIG. 1, a rectifier block R rectifies an input alternating-current voltage and an input capacitor Cin removes high-frequency components. When a switch 10 is turned on, electromagnetic energy accumulates in an inductor L. When the switch 10 is turned off, the energy in the inductor L is transferred to a capacitor Cout via a rectifying diode D and a power supply voltage is supplied to a load. When the switch 10 is turned on again, the diode D is cut off and the power supply voltage is output from the capacitor Cout. A one-cycle-control IC (OCC circuit) 20 is input with voltages of an input current detection resistor Rs and output voltage detection resistors R1 and R2 and controls the duty cycle of the switch 10 such that the AC input current comes to have the same phase as the AC input voltage.
However, since it is assumed that the PFC converter of Japanese Unexamined Patent Application Publication No. 2006-513682 will operate in a continuous current mode, if the PFC converter operates in a discontinuous current mode when the load is light, distortions are generated in the input current.
Furthermore, the PFC converter of Japanese Unexamined Patent Application Publication No. 2006-187140 is a multiphase PFC converter and is assumed to operate in a discontinuous current mode, and provided that the on period of switching is made constant throughout a single period of the commercial alternating-current power supply, the input current comes to have the form of a sine wave even when detection and control of the current is not performed. However, in reality, if a state arises in which the load increases and the PFC converter operates in a continuous current mode, distortions are generated in the input current.